1. Field of the Invention
The present invention generally relates to a method of manufacturing a permanent magnet from an alloy containing a rare earth metal and a ferrous component. More specifically, the present invention relates to a method of manufacturing a permanent magnet of a type having a high residual flux density and a high thermal stability, wherein flakes of an alloy containing a rare earth metal and a ferrous component, which is obtained by the use of a melt quenching process, is employed as the starting material.
2. Description of the Prior Art
As disclosed in the U.S. Pat. No. 4,802,931, a flaky alloy containing a rare earth metal and a ferrous component, obtained by the use of a melt quenching process, is known to have a relatively high coercive force and is currently attracting attention as a material for a permanent magnet. To produce this flaky alloy, the melt quenching process is carried out at a cooling rate of, for example, 10.sup.4 .degree. C./sec or higher down from a high temperature melt state with a portion thereof frozen in the melt state. This flaky alloy so obtained is an alloy in non-equilibrium having both an amorphous phase and a magnetic phase expressed by R.sub.2 TM.sub.14 B wherein R represents at least one of rare earth metals, and TM represents either Fe or Fe which is partially substituted by Co. If during the manufacture of the flaky alloy a heat treatment is effected at a temperature higher than the crystallization temperature under an inert atmosphere containing, for example, Ar gas according to a particular requirement, the flaky alloy wherein a R.sub.2 TM.sub.14 B phase is randomly aggregated can be obtained. In particular, if the grain size of the R.sub.2 TM.sub.14 B phase is adjusted to 40 to 400 nm, the maximum intrinsic coercive force can be obtained based on the composition of the alloy, readily reaching the level of a practically utilizable permanent magnet. However, the flaky alloy has a thickness generally within the range of 20 to 30 .mu.m and cannot therefore be used directly as a material for the permanent magnet. Accordingly, the alloy flakes are required to be prepared into an aggregate (billet) of any desired shape by the use of any suitable means while the flakes are interlocked with each other (compacted). As a means for interlocking the alloy flakes, use may be made of a suitable synthetic resin or of a hot press or a two-stage hot press.
A resin-bonded magnet of 80% relative density (volume % relative to full density) manufactured by heat-treating the flaky alloy obtained by the use of the melt quenching process, for example, the flaky alloy of Nd.sub.13 Fe.sub.83 B.sub.4, at a temperature higher than the crystallization temperature and having the grain size of the Nd.sub.2 Fe.sub.24 B phase which is adjusted to a value within the range of 40 to 400 nm, exhibits 6.1 kG in residual flux density, 15 kOe in intrinsic coercive force with its temperature coefficient being -0.42%/.degree. C., and 310.degree. C. in curie point. In this case, the flakes of the alloy are interlocked with each other by the use of a synthetic resin and, therefore, it is not difficult to cause it to have a relative density higher than 80%. Accordingly, the magnetic characteristics of the resin-bonded magnet referred to above can hardly be enhanced.
On the other hand, the hot-pressed magnet of 98 to 99% relative density wherein the flakes of the alloy of Nd.sub.13 Fe.sub.83 B.sub.4 have been interlocked with each other with no resin binder employed exhibits 7.9 kG in residual flux density, 16 kOe in intrinsic coercive force with its temperature coefficient being -0.47%/.degree.C., and 310.degree. C. in curie point. Therefore, this hot-pressed magnet can have high magnetic characteristics as compared with those of the resin-bonded magnet, if it is rendered to be of a high density. However, of the three factors, including the intrinsic coercive force, the temperature coefficient of the intrinsic coercive force and the curie point, which remarkedly affect the thermal stability represented by non-reversible demagnetization, the temperature coefficient of the intrinsic coercive force is somewhat high and the level of the residual flux density is lower by about 10 to 30% than the residual flux density of 9.0 to 11.3 kG exhibited by an Sm-Co sintered magnet manufactured according to powdery metallurgy.
A two-stage hot-pressed magnet wherein the hot-pressed magnet of 98 to 90% relative density made of the flaky alloy of Nd.sub.13 Fe.sub.83 B.sub.4 by the use of the melt quenching process is subjected to a die upsetting exhibits 11.8 kG in residual flux density, 13 kOe in intrinsic coercive force with its temperature coefficient being -0.60%/.degree.C., and 310.degree. C. in curie point. This two-stage hot-pressed magnet can have high magnetic characteristics as compared with those of the hot-pressed magnet by the utilization of the upset forging technique and, in particular, the level of the residual flux density thereof exceeds that of the Sm-Co sintered magnet manufactured according to powdery metallurgy. However, of the three factors, including the intrinsic coercive force, the temperature coefficient of the intrinsic coercive force and the curie point, which remarkedly affect the thermal stability of the magnet represented by non-reversible demagnetization, the temperature coefficient of the intrinsic coercive force is somewhat high and the level of the residual flux density is lowered by about 12 to 13% and the temperature coefficient thereof is increased by about 143%. This means that, even though an extremely high residual flux density is secured, the thermal stability of the magnet such as the non-reversible demagnetization is lowered. Accordingly, the application of the two-stage hot-pressed magnet in various motors or actuators which are generally operated, for example, under a high temperature limited in view of the limited temperature at which they are utilized, and therefore, there has been no way other than to use the Sm-Co sintered magnet of a composition containing Sm and Co, which are more expensive than the permanent magnet containing B and Fe as its principle component which is manufactured with resourceful light rare earth metals such as Nd and Pr.
A method of manufacturing the two-stage hot-pressed magnet known in the art comprises a step of filling the flaky alloy, obtained by the use of the melt quenching process and containing a rare earth metal and a ferrous component, in a molding cavity defined in a mold made of, for example, graphite and preheated to about 700.degree. C. in an inert atmosphere containing an Ar gas or in a vacuum atmosphere, and a step of applying one directional pressure when the alloy flakes are heated to a desired temperature by the heat conduction from the mold or by the application of a high frequency heating source. In other words, this method of making the two-stage hot-pressed magnet requires a heating temperature of 600.degree. to 900.degree. C. and a pressure of 1 to 3 ton/cm.sup.2. The subsequent hot-pressing is carried out with the use of a mold having a relatively large surface area. In general, this subsequent hot-pressing requires the use of the heating temperature of about 700.degree. C. and the pressure of 0.7 to 1.5 ton/cm.sup.2 . This method requires a precise control of the heating temperature and the applied pressure in coordination with time. However, since it is heated to a temperature higher than the crystallization temperature of the R.sub.2 TM.sub.14 B phase, the R.sub.2 TM.sub.14 B phase of the alloy flakes containing the rare earth metal and the ferrous component tends to become coarse. Accordingly, the grain size of the flaky alloy has to be reduced as compared with the size represented by the intrinsic coercive force based on the composition of the alloy.
As hereinbefore discussed, the permanent magnet made of the flaky alloy containing the rare earth metal, for example, B, and the ferrous component, for example, Fe with the use of resourceful light rare earth metals such as, for example, Nd and Pr can give a higher residual flux density, depending on a method for the manufacture thereof, than the Sm-Co sintered magnet containing the expensive Sm and Co. However, the permanent magnet made of the flaky alloy referred to above is susceptible to reduction in intrinsic coercive force or increase in temperature coefficient of the intrinsic coercive force and has therefore a problem in that, due to the reduction of the intrinsic coercive force or the increase of the temperature coefficient thereof, the thermal stability represented by the non-reversible demagnetization tends to be adversely affected. Also, the manufacturing method is complicated with a difficulty involved in precise control and machinability and, therefore, the yield tends to be lowered when it is used as material for the practically utilizable permanent magnets.